1. Field of the Invention
The present invention relates to an apparatus for producing a single crystal of semiconductor material in accordance with the Czochralski method.
2. The Prior Art
Such an apparatus is described in U.S. Pat. No. 5,316,742. It includes a crucible which is filled with molten semiconductor material. After immersion of a seed crystal, a rod-shaped single crystal is pulled from the melt. The crucible rests on an axially raisable and lowerable shaft which is raised while the single crystal is being pulled. This movement ensures that the axial position of the surface of the melt remains unchanged, even though the volume of the melt continuously decreases as a result of the growth of the crystal. An important feature of an apparatus of the above mentioned type is a heat shield which shields the growing single crystal against heat radiation which is radiated mainly by the crucible walls and a radiant heating element arranged laterally around the crucible. The heat shield held coaxially with respect to the single crystal extends close up to the melt surface and gives rise, in the region of the crystallization front of the single crystal, to a steep axial temperature gradient.
The temperature gradient is distinguished by a sharp drop in temperature between the crystallization front and the solid single crystal phase and is largely responsible for it being possible to pull the single crystal at a high rate. If, in addition to the heat shield, a cooling means is provided which effects cooling of the growing single crystal, it is possible to achieve particularly high pulling rates. Known cooling means comprise a duct system which surrounds the single crystal and has a liquid coolant flowing through it.
Embodiments in which the cooling means is extended as far as close up to the surface of the melt do, however, involve a high safety risk. A fault or failure of the system controlling the travel of the crucible may result in the cooling means being partially immersed in the melt and being destroyed, accompanied by coolant escaping and passing into the hot melt. The consequences of such an accident, which cannot be precluded, can be limited to an acceptable extent only by extensive and expensive safety measures. Embodiments in which the cooling means cannot be immersed in the melt even at maximum crucible travel do avoid the safety risk described. However, the distance between the cooling means and the surface of the melt remains comparatively large, and the effect of the cooling on the axial temperature gradient in the region of the crystallization front remains small.